The present invention relates to permeation installations.
The technique or process of permeation permits the separation of a gas from a mixture of gases in gaseous phase with the help of porous walls. This technique consists in applying under a relatively high pressure of the order of several tens of bars, the gaseous mixture in the environment of a bundle of hollow fibers produced from polymer of a particular type. Under the influence of the pressure and because of the nature of the material, the molecules of a gas will be adsorbed selectively by the material constituting the hollow fibers, passing through the pores of these porous fibers and will be recovered by desorption within the channel of very small size existing in these fibers. On the other hand, the gas or gases corresponding to the other molecules will not pass or will very little pass through the porous wall and will remain outside the bundle of hollow porous fibers.
In certain embodiments, a so-called sweeping gas flow, of a different composition than the permeate, is introduced within the fibers, from the side opposite that for recovery of the permeate. This injection has for its object to increase the yield by recuperation of the gas preferentially passing through the fibers.
This embodiment somewhat complicates the technique of permeators and is not described in what follows. The invention can also be applied to this type of embodiment.
In the accompanying FIG. 1, there is schematically shown a permeator. It comprises an external chamber 10 resistant to pressure, within which is mounted a bundle 12 of hollow porous fibers. The gaseous mixture is introduced by the nozzle 14 disposed at the lower end of the chamber 10. The gas under pressure surrounds the bundle of hollow fibers. The fraction of the gas which passes through the hollow wall penetrates the channels of the fibers and is recovered at one end of these latter in a chamber 16, the latter being connected to an outlet conduit 18 for the fraction of the gaseous mixture having passed through the wall of the fibers and which will ultimately be called permeate and indicated by the letter P. On the other hand, the fraction of the gaseous mixture which did not pass through the wall of the fibers is recovered, preferably with a tube provided with perforations 20 which extends axially along the bundle of fibers 12. This fraction of the gaseous mixture leaves the chamber 10 by the nozzle 22 connected to the perforated tube 20. The fraction of the gaseous mixture that did not pass through the wall of the hollow fibers will ultimately be called non-permeate and indicated by the letter R.
In another embodiment, the gaseous mixture is introduced via the central tube 22 and the non-permeate is recovered in the chamber 20.
As already indicated, the pressure of the gaseous mixture is relatively high, typically in the order of several tens of bars. It is thus necessary that the external chamber 10 with resistance to pressure have a relatively great wall thickness and the different nozzles such as 18 and 22 passing through this wall must also be made precisely to maintain the resistance to pressure of the chamber 10.
It will be understood that it is thus interesting to have several permeators 12 within the same pressure resistant chamber 10. This has already been proposed, particularly in U.S. Pat. No. 4,874,405, which discloses a permeation module consisting of several individual permeators disposed one above the other, these permeators being disposed in a same pressure resistant chamber.
However, it appears that, for reasons both technical and economical, it is difficult to have more than three permeators one above the other within a same chamber. However, there exists a certain number of situations in which it is desired to be able to process volumes of gaseous mixture with relatively high flow rates which are not compatible with the use of three permeators disposed one above the other.
To solve this problem, it could be envisaged to arrange within a same pressure chamber (shown at 24 in the accompanying FIG. 1A), several permeation ensembles, such as 26, disposed one beside the others. In FIG. 1A, there is also shown the inlet nozzle 28 for the gaseous mixture to be treated in the pressure chamber 24. However, it is important in such an installation that each permeation module be supplied by a predetermined gaseous flow rate, departing from this flow rate gives rise to a very great decrease in the output of the installation. To solve this problem, there can be envisaged provision of different permeation modules 26 in a pressure resistant chamber 24 of large dimension, such that the flow rates for each permeation module will be substantially the same. It will be understood, however, that such a solution is unacceptable because it leads to a very great increase in the cost of the installation because of the large dimensions of the pressure resistant chamber 24 and hence in particular the increase of the wall thickness of this chamber.
An object of the present invention is to provide a permeation chamber in which several permeators or several permeation modules are disposed one beside the other within a single pressure resistant chamber whose dimension is reduced whilst permitting an overall higher output from this installation.
To achieve this object, according to the invention, the permeation installation comprises:
a single pressure resistant chamber,
at least two permeation modules disposed within said chamber, each module being constituted by at least one permeator formed of hollow fibers with a porous wall and being disposed within an envelope provided with perforations placing in communication the external portion of each module and the common gaseous circuit,
means to supply said installation with a gaseous mixture to be processed,
means to recover the fraction of the gaseous mixture that has passed through the wall of said fibers;
means to recover the fraction of the gaseous mixture that has not passed through the wall of said fibers.
It will be understood that, thanks to the interposition of the perforated envelopes constituting pressure drops in the common gaseous circuit, the modules are supplied with substantially equal flow rates for the different modules and with a good distribution of the flows for each module, thereby permitting the optimum operation of each permeation module and accordingly the optimum operation of the unit. It will be moreover understood that the perforated envelopes are disposed at the interface of two gaseous media whose pressures are not very different. These envelopes can therefore have a relatively simple mechanical construction.
According to a first embodiment, the perforators are interposed in the supply circuit of the gaseous mixture to the permeation modules. According to a second embodiment, the perforations are interposed in the recovery circuit of the fraction of the gaseous mixture that has not passed through the walls of the fibers of the permeators (non-permeate).
It will be understood that, in these two cases, the pressure drop which results permits substantially equalizing the gaseous flow rate in all the permeation modules.
According to a first embodiment of the installation, each permeation module has a generally cylindrical shape, each envelope has a cylindrical shape surrounding said permeation module over all its axial length and said cylindrical envelope is perforated in its lateral portion and is closed at each of its ends by a closed wall.
According to a second embodiment of the invention, each permeation module has a generally cylindrical shape, each permeation module is surrounded over all its length by an imperforate cylindrical wall and an imperforate plate closes one end of the imperforate cylindrical wall, another end plate having said perforations.
Preferably, the pressure drop created by said perforations is comprised between 10 and 90% of the total pressure drop between the inlet of the installation and the outlet for the fraction of the gaseous mixture that has not passed through the wall of the fibers, and preferably between 15 and 60%.